Biology Reference
In-Depth Information
1. INTRODUCTION
Haem is one of the most widely used prosthetic groups in biological
systems in both prokaryotes and eukaryotes because haem can provide a
variety of biological functions including oxygen transport/storage, electron
transfer, enzymatic oxidation and oxygenation of various substrates, enzy-
matic dehydration of substrates, and signal transduction. Though the basic
structure of haem is identical among these haem proteins, the chemical
and physical properties of haem can be tuned diversely by changing in
the coordination structure of haem and in haem environmental structures
including interactions between haem and surrounding amino acid residues.
For example, electronic properties of haem including redox potentials can
be tuned by changing in the coordination structures (coordination number
and axial ligands) of haem and/or of hydrophobicity/hydrophilicity of haem
pocket. External ligand-binding properties can also be controlled by chang-
ing in the coordination and haem environmental structures. Thus, specific
interactions between haem and protein matrix in different ways provide
diversities of biological functions for haem proteins.
In the haem-based sensor proteins, haem acts as the active site for sensing
external signals such as diatomic gas molecules or redox changes ( Aono,
2003, 2011; Gilles-Gonzalez & Gonzalez, 2005; Roberts, Kerby,
Youn, & Conrad, 2005 ). It makes sense that gas sensor proteins adopt haem
as the active site for sensing O 2 , CO, and NO because haem can bind these
gas molecules reversibly. These sensor proteins are involved in the regula-
tion of gene expressions, chemotaxis, and synthesis/degradation of second
messenger molecules, where these gas molecules act as physiological effec-
tors. They are usually multi-domain proteins consisting of regulatory
domain(s) along with a sensor domain in the same molecule, while sensor
proteins consisting of only a single sensor domain also exist. The sensor
domains/proteins regulate biological functions of regulatory domains/
proteins in response to their physiological effectors, for which intra- and/
or intermolecular signal transduction play important roles.
The binding of a physiological gasmolecule to haem is the first step for sens-
ing of the cognate physiological effector and of functional regulation of the
haem-based sensor proteins. Upon binding of O 2 , CO, orNO to haem, a con-
formational change around the haemwill be induced by interactions between
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